The long term objective of this program of research is to investigate the role social awareness in the development of communication in autistic persons. The syndrome of autism is characterized by marked impairment of both language and social behavior, and even in those autistic persons who do develop language, social communication remains abnormal. This project rests on the assumption that language and social behavior are closely intertwined in development, and that in disorders of the autistic spectrum this developmental relation is disrupted, resulting in characteristic impairments of language pragmatics. In addition it is recognized that among persons who show characteristics of autism, there is a wide range in severity of impairment in social, linguistic and intellectual functioning. The proposed project will focus on the autistic person's developing awareness of other persons' affective states and knowledge states in communicative interactions. It is argued that these areas of awareness are critically important to appropriate functional communication, and that they are of special difficulty in autism. A group of autistic persons (3-16) and a mental age-matched group with Down syndrome will be studied at yearly intervals for 4 years. The research will have the following specific aims: 1) to examine longitudinally the awareness of other persons' affective states and knowledge states during conversational interactions in these groups 2) to examine the developmental relations among social, communicative, and intellectual functioning across a broad range of developmental levels in these groups. It is hypothesized that all persons studied, including those at very low levels of development, will show some gains in social/communicative outcome. It is further hypothesized that differences between mental age- matched Autism and Down syndrome groups will be present in tasks measuring referential communication skills and tasks measuring interpretation of affect. Results are expected not only to better define the social communication deficits associated with disorders of the autistic spectrum, but to improve understanding of the growth of specific skills in this group.